Slander is oral speech; libel is the one you're looking for. My understanding is that a statement has to be false in order to be libel, but I wouldn't advise the poster to take legal advice from a rando on HN like me.
Peter Parker: Spider-Man wasn't trying to attack the city, he was trying to
save it. That's slander.
J. Jonah Jameson: It is not! I resent that! Slander is spoken. In print,
it's libel.